Organic light-emitting diodes are a device which injects a hole and electron into its emission layer of organic thin film to transform the electrical energy into optical energy and thereby to emit light. Light emitting displays provided with an organic light-emitting diode as a light-emitting device constituting a pixel (hereinafter referred to as “OLED display”) need no auxiliary light source, e.g., back light, and can be thinner and lighter than a non-emissive type display represented by a liquid crystal display. Moreover, it is shorter in response time for displaying, which is its another characteristic.
Various organic light-emitting diode structures have been proposed, e.g., a three-layer structure comprising, in addition to an electrode working as the anode and another electrode working as the cathode, a hole-transporting layer, an emission layer and an electron-transporting layer in this order between the anode and cathode from the anode side; two-layer structure with one layer working as the light-emitting and electron-transporting layers; and still another structure further having a hole-injecting layer between the anode and hole-transporting layer. A layer having a emission layer formed between an anode and cathode is hereinafter referred to as a “light-emitting thin-film layer” for convenience.
Any structure which emits light from one side of its emission layer is normally provided with an optically transparent electrode (anode or cathode) of a light-transmitting, electroconductive material on the light-emitting side, with the other electrode being of a light-reflective metal to reflect light.
In the above structure, part of light radiated from the light-emitting thin-film layer is directly emitted outwards after being transmitted by the optically transparent electrode, and the remainder is emitted outwards after being reflected by the light-reflective electrode on the back. Therefore, it is essential for an OLED display provided with such an organic light-emitting diode to have the light-reflective electrode of high reflectance, in order to increase emitted light and thereby display bright images.